Speed reducers for use in the field of a robot are mainly classified into two kinds: a harmonic speed reducer and a cycloidal pin gear speed reducer. The harmonic speed reducer, which was used as a main device for transmission of a robot, realizes speed-reduction motion through elastic deformation of a flexible gear, which has larger elastic deformation and also has a large elastic return difference caused thereby, thus inevitably affecting the accuracy of motion of the robot. In the case of transmitting a larger load, meshing of a deformative flexible gear and a rigid gear is not conjugate tooth profile meshing, thus the service life for ensuring the precision is short, and the meshing of the deformative flexible gear and the rigid gear can only be applied on a light-load robot or middle-light-load robot instead of a heavy-load robot. Compared with the harmonic transmission speed reducer, a Japanese Rotary Vector (simply called RV) speed reducer has a series of advantages of a small size, light weight, a large transmission ratio, large rigidity, high precision and efficiency, smooth operation, small return difference, high motion precision, and the like, and is applicable for use in a heavy-load robot. The newest Japanese product of a RV speed reducer allows, through alternation of an external ring of a bearing, the size of the speed reducer to be further reduced, and consequently a body interface of the robot also becomes small, which represents a trend that the speed reducer for use in the robot is developed in a smaller and better direction. However, in the case that the size of the speed reducer is constant, a transmission ratio of the RV speed reducer increases as the number of teeth of a pin gear, and dimensions of a cycloidal single tooth and a gear pin become small, which affects the further improvement of load-bearing capacity of the speed reducer. Moreover, the RV speed reducer is high in machining precision requirement and cost, which restricts popularization and application of the RV speed reducer. Particularly, it is very difficult to meet machining precision with which two eccentric shafts are disposed at a difference of 180 degree, however the precision of the RV speed reducer directly affects transmission precision and efficiency of the speed reducer. A large speed ratio of the RV speed reducer comes from planetary speed-reduction motion and cycloidal pin gear speed-reduction motion of the RV speed reducer, which form composite two-level speed-reduction, and large speed-reduction ratio may be realized with series connection of Japanese FA speed reducers or series connection of other speed-reducing devices, but the axial dimension is multiplied as the series connection of the speed reducer, which can not meet the requirement of small size, high rigidity, and high efficient load-bearing with respect to the speed reducer for use in the robot. Therefore, the reducer with a less teeth number, a large speed ratio, a small size, high load-bearing, high efficiency, and long service life is a bottleneck that the robot is developed in a direction of a small size, high efficiency, and high load-bearing, which needs to be solved urgently.